1. Field of the Invention
The present invention is directed to a method for magneto-optical readout of data stored in a magnetic storage medium, as well as to a magneto-optical readout head for effecting such readout, and to a method for making such a magneto-optical readout head.
2. Description of the Prior Art
Data can be magnetically stored in a variety of magnetic media, such as tapes and discs of the type for computer data storage, and video and audio discs and tapes for the storage of entertainment data.
A number of technologies are available for constructing a readout head for retrieving the magnetically stored data from such media. One known technology is that of inductive thin film heads. The inductive thin film head is the basic component in the playback of recorded signals of all types. A head of this type has a magnetic thin film core which senses the changing magnetic flux from a recorded tape or disc. The type of core used in inductive thin film heads currently being widely manufactured is formed of a flux-conducting magnetic material with a very high permeability, the core being provided with a winding and having a small gap therein. The gap collects or senses the available flux from the recorded track, and the core interacts with the winding so that a voltage, corresponding to the recorded data, is produced across the ends of the winding. Inductive thin film heads of this type are described, for example, in Integrated Magnetic Recording Heads, Lazzari et al, IEEE Trans. Magn., Vol. MAG7(1), March, 1971, pages 146-150; Magnetic Instability Of Thin Film Recording Heads, IEEE Trans. Magn., Vol. MAG30(2), March 1994, pages 375-380; and The Complete Handbook Of Magnetic Recording, 4th Ed., Jorgensen et al, TAB Books, 1995, pages 238-262.
A problem with conventional inductive thin film heads is that it is very difficult to increase the information density handled by such heads, because the distance between the poles of such a head is finite, and cannot be completely eliminated without destroying the intended operation of the head. Moreover, a gap interface between the gap surfaces and the ambient environment results in a signal loss, usually referred to as spacing loss. Although great strides have been taken towards miniaturization of such heads, practical constraints impose the necessity of a very precise mechanical design and exacting manufacturing techniques for ultra-high density storage. Thin film inductive heads also exhibit a poorer carrier-to-noise ratio (CNR) then other head technologies. Moreover, the existing state of the technology relating to thin film inductive heads makes it difficult to manufacture multi-track heads which can simultaneously read information from a number of parallel recording tracks without mistracking.
Another known head technology is the so-called giant magneto-resistive (GMR) head. This type of head is manufactured from a magneto-resistive material which makes use of phenomena which occur when thin magnetic layers (1-3 nm) of transition metals (Fe, Co, Ni) are separated by ultra thin (a few angstroms) of non-magnetic metal (Cr, Cu, Ag, Au). Giant magneto-resistive heads are described, for example, in "Giant Magneto-Resistance Materials And Their Potential As Readhead Sensors," White, Trans. Magn., Vol. MAG-30(2), March 1994, pages 346-352; "GMR Multi-Layers And Head Design For Ultra-High Density Magnetic Recording," Parker et al., TMRC'95, IEEE Trans. Magn., Vol. 32, pages 135-141; and "The Complete Handbook Of Magnetic Recording, 4th Ed., Jorgensen, TAB Books, 1995, page 193.
The production of readout heads according to GMR technology requires ultra-precise (nanometric) manufacturing techniques which results in a small production yield, thereby effecting the economic viability of this approach. Furthermore, the subnanometric nature of the fabrication makes it very difficult to maintain consistent parameters from one head cell to another in multi-track assemblies. The sensitivity coefficient (resistance per unit of external field, .DELTA..rho.=.DELTA..OMEGA./H.sub.ex) requires very precise thickness control, with tolerances of typically .+-.3 to 4 .ANG.. GMR heads also exhibit a problem associated with inter-diffusion between the ultra-thin non-magnetic conductor layer (usually a few angstroms) and the adjacent magnetic layers. Another problem associated with GMR heads (which also exists with thin film inductive heads) is that an Eddy-current limitation occurs at higher data rates, which limits the data handling rate of such heads.
A magneto-optical recording and playback technology has been developed by Thomson-CSF, among others. This technology employs a matrix magnetic head to write multiple tracks (100 to 1000 tracks) in parallel. Readout takes place using a magneto-optical head employing the Kerr magneto-optical effect (hereinafter referred to simply as the Kerr effect). The Kerr effect is a known phenomenon whereby changes in the optical properties of a reflecting surface of a ferromagnetic substance are produced when the substance is magnetized. This phenomenon applies particularly to the elliptical polarization of reflected light, when the ordinary rules of metallic reflection would produce only plane polarized light. In this type of head technology, the tape is read with a wide magnetic head which reads all of the tracks in parallel. The magnetic field picked up with the head is used to modulate polarized light, using the Kerr effect, which changes the polarization angle of the light. A light beam is directed through a fixed polarizer onto a CCD line detector, with one pixel for each track. An advantage of this technology is that many tracks can be recorded and read in parallel at the same time, without guard bands between the tracks. A disadvantage associated with this technology is that thus far the Kerr element in the read head has proven to have performance limitations associated therewith.
Heads of the above type employing the Kerr effect are described, for example, in "Toward The Multi-Track Digital Video Tape Recorder," Maurice, MORIS 91, J. Magn. Soc. Jpn., Vol. 15, Supp. No. S1, 1991, pages 389-394; "The Kerr Head: A Multi-Track Fixed Active Head," Maillot et al, Intermag '92, IEEE Trans. Magn., Vol. 28, No. 5, September 1992; "Longitudinal Kerr Effect Enhancement Of A 384 Track Head For High Data Rate Readout," Le Texier et al, MMM Conf. '93, Houston Tex.; and U.S. Pat. Nos. 5,282,104; 5,227,938; 5,189,579; 5,167,062; 5,157,641; 5,123,156; 5,093,980; 5,050,027; 4,897,747 and 4,275,428, all assigned to Thomson CSF; and U.S. Pat. No. 5,365,391 assigned to Sony Corp.
The known magneto-optical head developed by Thomson CSF in accordance with the above references has the disadvantage of a small CNR.
Given a magnetic field of a strength typical in this technology, the rotation angle in the polarization plane is small in the Thomson CSF head, typically only approximately 0.35.degree. at 633 nm. The Thomson CSF head also exhibits cross-talk between adjacent tracks of a head of multi-track design, as a result of the necessity of employing a sensitive surface which is not mono-crystalline. Control of the magnetic properties of the very thin Sendust.RTM. (FeAlSi) gap layer remains a significant problem with regard to manufacturing consistency. Additionally, the optical properties of this Sendust.RTM. are complex, and contribute to the difficulty of optimizing the optical path. The optical path in the Thomson CSF head is therefore far from ideal, both from the point of view of optical efficiency and the point of view of optimizing the sensitivity to signal detection. Lastly, magnetic noise, resulting from the Barkhausen effect, contributes to a reduction of the CNR at larger illuminated areas.
Another magneto-optical head has been proposed by Garnetec. This head uses the Faraday effect. The Faraday effect is a known phenomenon whereby the polarization of a beam of linearly polarized light is rotated when the light passes through a substance in the direction of an applied magnetic field. This effect results from Faraday birefringence, which is the difference in the indices of refraction of left and right circularly polarized light passing through a substance parallel to an applied magnetic field. In the head proposed by Garnetec, the Faraday effect is produced by a transparent magnetic thin film with initial in-plane magnetization, which functions as a Faraday rotator. The polarization rotation produced by the Faraday effect is more pronounced than that produced by the longitudinal Kerr effect. The head proposed by Garnetec has a side which faces away from the magnetic storage medium which has a curved shape (convexity) which acts to magnify the image of the domain structure of the transparent Faraday effect film. The use of such a magnifier improves resolution considerably, if the magnifier material has a high refractive index.
The head proposed by Garnetec is described in co-pending U.S. patent application Ser. No. 08/842,286 filed on Apr. 23, 1997 ("Magneto-Optical Head For Information Reading," assigned to Garnetec), and a discussion of related physics is found in "Method For Observation And Measurement Of The Velocity Of Bubble Propagation In Thin Ferrogarnet Films," II'Yashenko, Physica Status Solidi, Vol. 36, 1976, pages K1-K6.
Garnetec has also proposed a multi-track readout head wherein the Faraday effect is used twice, but becomes smaller at ultra high density recording. This is because at very high optical resolution, with a transition length of only 0.1 .mu.m at a wavelength of 630 nm, it is necessary to decrease the thickness t of the magneto-optically active thin film (usually bi-substituted ferrite-garnet film) in order to increase the optical resolution. In this case, the total Faraday resolution .PSI..sub.F is also decreased, because .PSI..sub.F =2.theta..sub.F .multidot.t, where .theta..sub.F is the Faraday rotation coefficient. For transition lengths of 0.15 .mu.m and less, a magneto-optical thin film of not more than 0.2 .mu.m would be required. The total Faraday rotation .theta..sub.F for the best magneto-optically active films are less than 1.degree. at a wavelength of 633 nm. Moreover, the polarizing resolution of the head is decreased, and consequently CNR is decreased.